A basic function of an uninterruptible power system (“UPS”) is to ensure continued delivery of power to loads under a variety of fault conditions. With reference to the block diagram of FIG. 1, for example, a UPS 100 may comprise a first input 102 for receiving energy from a primary power source 103, such as an AC utility source; a second input 104 for receiving energy from a second power source 105, such as a battery or an AC generator; and an output 106 for delivering energy to loads 112. In some embodiments the second power source 105 may be included within the UPS 100. In operation, power for loads 112 may be derived from the primary power source 103 when primary power source quality is acceptable (e.g., when the source is within pre-defined limits of voltage and frequency) or may be derived from the backup power source 105 when the primary power source quality is not acceptable. In many systems, it is important the switch between one source and the other be done in a manner that is essentially transparent to the loads. For example, the power sources may be single or three-phase AC sources or DC sources.
Various UPS configurations are currently known. One configuration, referred to herein as a double-conversion UPS, is illustrated in the block diagram of FIG. 2. The double-conversion UPS 100a may, e.g., receive primary power from a three-phase AC utility source 103 and receive backup power from a bank of storage batteries 105a. A rectifier-charger circuit 114 converts the three-phase AC input into DC and an inverter circuit 116 converts the DC back into a three-phase AC output for delivery to loads 112. A controller 118 may monitor various system parameters and control the rectifier-charger circuit 114 and the inverter circuit 116 as a means of providing uninterrupted power flow to the loads 112; the controller may also control the inverter 116 to regulate the voltage and frequency of the AC output delivered to the loads.
Another UPS configuration, referred to herein as a line-interactive UPS, is shown in FIG. 3. The line interactive UPS 100b may, e.g., receive primary power from a three-phase AC utility source 103 and receive backup power from a backup AC generator 105b. The backup AC generator may, e.g., be a flywheel motor/generator of the kind described in Clifton et al, Energy Storage Flywheel Emergency Power Source and Methods, U.S. Pat. No. 5,932,935, issued Apr. 11, 1997 which is hereby incorporated by reference in its entirety. Each phase of the line-interactive UPS 100b may comprise a static AC switch 122 and a backup power conditioner 130. With reference to FIG. 4, a static AC switch 122 may comprise a pair of back-to-back SCRs 161,162. The backup power conditioner may comprise a flywheel inverter 128, a storage capacitor 126 and a utility converter 124. A controller 120 monitors the various inputs and outputs and controls the static AC switch 122 and the backup power conditioner 130 in order to provide uninterrupted power flow to the loads 112. Operation of a line-interactive converter is described in detail in Operation and Performance of a Flywheel-Based Uninterruptible Power Supply (UPS) System, White Paper #108, published by Active Power Inc., Austin, Tex., 78758, USA which is hereby incorporated by reference in its entirety. Under “normal” operating conditions (as used herein, “normal operation” or “normal operating conditions” refer to operation or conditions under which the primary source is within acceptable operating limits of voltage and frequency and power is being delivered from the AC utility source to the loads), the static AC switch 122 is ON and three-phase power is delivered from the AC utility source 103 to the loads via the output three-phase bus 136; the controller 120 may also regulate the output three-phase bus voltage by controlling the flow of reactive power between the power conditioner 130 and the bus 136 via inductor 134.
Other known UPS topologies include, but are not limited to, Delta Conversion UPS, Rotary UPS and Hybrid UPS. Known backup energy sources include, but are not limited to, batteries, flywheel motor-generators, compressed air, fuel cells and fossil fuel powered motor-generator sets.
As shown in FIGS. 2 and 3, a UPS may comprise a bypass circuit 140 and the bypass circuit may, e.g., comprise a static AC switch of the kind shown in FIG. 4. When enabled, the bypass circuit 140 provides a direct connection between the primary power source and the loads.
Conversion efficiency during normal operation is an important UPS performance factor because higher conversion efficiency translates into reduced power loss and lower utility costs. Because the double-conversion UPS configuration processes utility power in each of two cascaded stages, its operating efficiency under normal operating conditions may be lower when compared, e.g., to a line interactive UPS, in which normal power flow is through a static AC switch. To improve normal operating efficiency, a double-conversion UPS may, under normal operating conditions, enable its bypass circuit 140, thereby allowing power to flow directly from the AC utility source 103 to the loads 112 and avoiding some of the losses associated with cascade power processing. This “eco-mode” of operation may improve normal conversion efficiency to a level comparable to the efficiency of a line-interactive converter; in doing so, however, the regulation and isolation advantages provided by the double-conversion topology are lost.
In operation, a UPS responds to a variety of fault conditions. For example, upon detection of an input undervoltage condition, a typical UPS may first respond by disconnecting the primary source and enabling power delivery between the secondary source 105 and the loads 112. Thereafter, the controller may monitor for an output fault (e.g., an output undervoltage condition), because presence of an output fault may indicate an overcurrent condition at the loads 112. If no output fault is detected, and the input fault persists, the loads may remain connected to the second source. If an output undervoltage fault is detected, however, the controller may disconnect the second source and activate a bypass circuit to directly connect the primary source 103 to the loads 112.